Editing IPCC:AR6/SROCC/Chapter-2 (section)

===== 2.3.2.1.1 Unstable slopes, landslides and glacier instabilities =====

Permafrost degradation and thaw as well as increased water flow into frozen slopes can increase the rate of movement of frozen debris bodies and lower their surface due to loss of ground ice (subsidence). Such processes affected engineered structures such as buildings, hazard protection structures, roads, or rail lines in all high mountains during recent decades (Section 2.3.4). Movement of frozen slopes and ground subsidence/heave are strongly related to ground temperature, ice content, and water input (Wirz et al., 2016 <sup>[[#fn:r447|447]]</sup> ; Kenner et al., 2017 <sup>[[#fn:r448|448]]</sup> ). Where massive ground ice gets exposed, retrogressive thaw erosion develops (Niu et al., 2012 <sup>[[#fn:r449|449]]</sup> ). The creep of rock glaciers (frozen debris tongues that slowly deform under gravity) is in principle expected to accelerate in response to rising ground temperatures, until substantial volumetric ice contents have melted out (Kääb et al., 2007 <sup>[[#fn:r450|450]]</sup> ; Arenson et al., 2015a <sup>[[#fn:r451|451]]</sup> ). As documented for instance for sites in the European Alps and Scandinavia for recent years to decades, rock glaciers replenished debris flow starting zones at their fronts, so that the intensified material supply associated with accelerated movement (Section 2.2.4) contributed to increased debris flow activity (higher frequency, larger magnitudes) or slope destabilisation (Stoffel and Graf, 2015 <sup>[[#fn:r452|452]]</sup> ; Wirz et al., 2016 <sup>[[#fn:r453|453]]</sup> ; Kummert et al., 2017 <sup>[[#fn:r454|454]]</sup> ; Eriksen et al., 2018 <sup>[[#fn:r455|455]]</sup> ).

There is ''high confidence'' that the frequency of rocks detaching and falling from steep slopes (rock fall) has increased within zones of degrading permafrost over the past half-century, for instance in high mountains in North America, New Zealand, and Europe (Allen et al., 2011 <sup>[[#fn:r456|456]]</sup> ; Ravanel and Deline, 2011 <sup>[[#fn:r457|457]]</sup> ; Fischer et al., 2012 <sup>[[#fn:r458|458]]</sup> ; Coe et al., 2017 <sup>[[#fn:r459|459]]</sup> ). Compared to the SREX and AR5 reports, the confidence in this finding increased. Available field evidence agrees with theoretical considerations and calculations that permafrost thaw increases the likelihood of rock fall (and also rock avalanches, which have larger volumes compared to rock falls) (Gruber and Haeberli, 2007 <sup>[[#fn:r460|460]]</sup> ; Krautblatter et al., 2013 <sup>[[#fn:r461|461]]</sup> ). These conclusions are also supported by observed ice in the detachment zone of previous events in North America, Iceland and Europe (Geertsema et al., 2006 <sup>[[#fn:r462|462]]</sup> ; Phillips et al., 2017 <sup>[[#fn:r463|463]]</sup> ; Sæmundsson et al., 2018 <sup>[[#fn:r464|464]]</sup> ). Summer heat waves have in recent years triggered rock instability with delays of only a few days or weeks in the European Alps (Allen and Huggel, 2013 <sup>[[#fn:r465|465]]</sup> ; Ravanel et al., 2017 <sup>[[#fn:r466|466]]</sup> ). This is in line with theoretical considerations about fast thaw of ice filled frozen fractures in bedrock (Hasler et al., 2011 <sup>[[#fn:r467|467]]</sup> ) and other climate impacts on rock stability, such as from large temperature variations (Luethi et al., 2015 <sup>[[#fn:r468|468]]</sup> ). Similarly, permafrost thaw increased the frequency and volumes of landslides from frozen sediments in many mountain regions in recent decades (Wei et al., 2006 <sup>[[#fn:r469|469]]</sup> ; Ravanel et al., 2010 <sup>[[#fn:r470|470]]</sup> ; Lacelle et al., 2015 <sup>[[#fn:r471|471]]</sup> ). At lower elevations in the French Alps, though, climate driven changes such as a reduction in number of freezing days are projected to lead to a reduction in debris flows (Jomelli et al., 2009 <sup>[[#fn:r472|472]]</sup> ).

A range of slope instability types was found to be connected to glacier retreat (Allen et al., 2011 <sup>[[#fn:r473|473]]</sup> ; Evans and Delaney, 2015 <sup>[[#fn:r474|474]]</sup> ). Debris left behind by retreating glaciers (moraines) slid or collapsed, or formed fast flowing water-debris mixtures (debris flows) in recent decades, for instance in the European and New Zealand Alps (Zimmermann and Haeberli, 1992 <sup>[[#fn:r475|475]]</sup> ; Blair, 1994 <sup>[[#fn:r476|476]]</sup> ; Curry et al., 2006 <sup>[[#fn:r477|477]]</sup> ; Eichel et al., 2018 <sup>[[#fn:r478|478]]</sup> ). Over decades to millennia, or even longer, rock slopes adjacent to or formerly covered glaciers, became unstable and in some cases, eventually collapsed. Related landslide activity increased in recently deglacierised zones in most high mountains (Korup et al., 2012 <sup>[[#fn:r479|479]]</sup> ; McColl, 2012 <sup>[[#fn:r480|480]]</sup> ; Deline et al., 2015 <sup>[[#fn:r481|481]]</sup> ; Kos et al., 2016 <sup>[[#fn:r482|482]]</sup> ; Serrano et al., 2018 <sup>[[#fn:r483|483]]</sup> ). For example, according to Cloutier et al. (2017) <sup>[[#fn:r484|484]]</sup> more than two-thirds of the large landslides that occurred in Northern British Columbia between 1973–2003, occurred on cirque walls that have been exposed after glacier retreat from the mid-19th century on. Ice-rich permafrost environments following glacial retreat enhanced slope mass movements (Oliva and Ruiz-Fernández, 2015 <sup>[[#fn:r485|485]]</sup> ). At lower elevations, re-vegetation and rise of tree limit are able to stabilise shallow slope instabilities (Curry et al., 2006 <sup>[[#fn:r486|486]]</sup> ). Overall, there is ''high confidence'' that glacier retreat in general has in most high mountains destabilised adjacent debris and rock slopes over time scales from years to millennia, but robust statistics about current trends in this development are lacking. This finding reconfirms, and for some processes increases confidence in related findings from the SREX and AR5 reports.

Ice break-off and subsequent ice avalanches are natural processes at steep glacier fronts. How climate driven changes in geometry and thermal regime of such glaciers influenced ice avalanche hazards over years to decades depended strongly on local conditions, as shown for the European Alps (Fischer et al., 2013 <sup>[[#fn:r487|487]]</sup> ; Faillettaz et al., 2015 <sup>[[#fn:r488|488]]</sup> ). The few available observations are insufficient to detect trends. Where steep glaciers are frozen to bedrock, there is, however, ''medium evidence'' and ''high agreement'' from observations in the European Alps and from numerical simulations that failures of large parts of these glaciers were and will be facilitated in the future due to an increase in basal ice temperature (Fischer et al., 2013 <sup>[[#fn:r489|489]]</sup> ; Faillettaz et al., 2015 <sup>[[#fn:r490|490]]</sup> ; Gilbert et al., 2015 <sup>[[#fn:r491|491]]</sup> ) .

In some regions, glacier surges constitute a recurring hazard, due to widespread, quasi-periodic and substantial increases in glacier speed over a period of a few months to years, often accompanied by glacier advance (Harrison et al., 2015 <sup>[[#fn:r492|492]]</sup> ; Sevestre and Benn, 2015 <sup>[[#fn:r493|493]]</sup> ). In a number of cases, mostly in North America and High Mountain Asia (Bevington and Copland, 2014 <sup>[[#fn:r494|494]]</sup> ; Round et al., 2017 <sup>[[#fn:r495|495]]</sup> ; Steiner et al., 2018 <sup>[[#fn:r496|496]]</sup> ), surge-related glacier advances dammed rivers, causing major floods. In rare cases, glacier surges directly inundated agricultural land and damaged infrastructure (Shangguan et al., 2016 <sup>[[#fn:r497|497]]</sup> ). Sevestre and Benn (2015) <sup>[[#fn:r498|498]]</sup> suggest that surging operates within a climatic envelope of temperature and precipitation conditions, and that shifts in these conditions can modify surge frequencies and magnitudes. Some glaciers have reduced or stopped surge activity, or are projected to do so within decades, as a consequence of negative glacier mass balances (Eisen et al., 2001 <sup>[[#fn:r499|499]]</sup> ; Kienholz et al., 2017 <sup>[[#fn:r500|500]]</sup> ). For such cases, related hazards can also be expected to decrease. In contrast, intensive or increased surge activity (Hewitt, 2007 <sup>[[#fn:r501|501]]</sup> ; Gardelle et al., 2012 <sup>[[#fn:r502|502]]</sup> ; Yasuda and Furuya, 2015 <sup>[[#fn:r503|503]]</sup> ) occurred in a region on and around the Western Tibetan plateau which exhibited balanced or even positive glacier mass budgets in recent decades (Brun et al., 2017 <sup>[[#fn:r504|504]]</sup> ). Enhanced melt water production was suggested to be able to trigger or enhance surge-type instability, in particular for glaciers that contain ice both at the melting point and considerably below (Dunse et al., 2015 <sup>[[#fn:r505|505]]</sup> ; Yasuda and Furuya, 2015 <sup>[[#fn:r506|506]]</sup> ; Nuth et al., 2019 <sup>[[#fn:r507|507]]</sup> ).

A rare type of glacier instability with large volumes (in the order of 10-100 million m 3 ) and high mobility (up to 200–300 km/h) results from the complete collapse of large sections of low-angle valley glaciers and subsequent combined ice/rock/debris avalanches. The largest of such glacier collapses have been reported in the Caucasus Mountains in 2002 (Kolka Glacier, ~130 fatalities) (Huggel et al., 2005 <sup>[[#fn:r508|508]]</sup> ; Evans et al., 2009 <sup>[[#fn:r509|509]]</sup> ), and in the Aru Range in Tibet in 2016 (twin glacier collapses with 9 fatalities) (Kääb et al., 2018 <sup>[[#fn:r510|510]]</sup> ). Although there is no evidence that climate change has played a direct role in the 2002 event, changes in glacier mass balance, water input into the glaciers, and the frozen regime of the glacier beds were involved in the 2016 collapses and at least partly linked with climate change (Gilbert et al., 2018 <sup>[[#fn:r511|511]]</sup> ). Besides the 2016 Tibet cases, it is unknown if such massive and rare collapse-like glacier instabilities can be attributed to climate change.

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